This invention relates generally to humidistats for maintaining relative humidity in a package and more particularly concerns a humidistat pad which serves both as a spacer in a package and as a humidistat.
A number of perishable commodities including food products and tobacco products, for example, retain their optimum freshness and desirability when stored in environments in which a predetermined relative humidity is maintained. Normally, in the case of food and tobacco products, it is desirable to maintain a relative humidity in the package which is higher than the ambient relative humidity. It may also be desirable, however, to maintain a relative humidity in the package which is less than the ambient relative humidity when the same food and tobacco products are stored in damp environments.
In general, the relative humidity in an enclosed, generally air tight package is maintained by providing a humectant, generally a water based solution, within the package in its liquid phase. The humectant solution vaporizes in the package producing water vapor in the package. When the partial pressure of water vapor in the package is equal to the vapor pressure of water for the humectant solution, an equilibrium is reached where the humectant gives off water vapor at the same rate as water vapor condenses back into the humectant liquid. Depending on the equilibrium vapor pressure of the water over the humectant solution selected, a predetermined relative humidity may be established in the enclosed package.
As recognized in the prior art, one important aspect of any liquid humidistat used in a package is confinement of the liquid phase so that the liquid does not come into direct contact with either the package or with the product packaged therein via wicking, gravity, etc.
Sabin U.S. Pat. No. 2,452,957 discloses a moisture control device for humidifying a conventional package of cigarettes. The humidistat includes a rectangular envelope made of uncoated regenerated cellulose or coated regenerated cellulose. Such materials are substantially waterproof, i.e. impermeable to liquid, but capable of transmitting vapor. Two grams of moistened Glauber's salt (sodium sulfate decahydrate) are sealed in the envelope to provide a source of water vapor while retaining the liquid solution from contacting either the package or the cigarettes.
Gailey U.S. Pat. No. 2,365,185 discloses a tobacco humidifying device. A textured sponge rubber sponge is saturated with water and attached to the lid of a tobacco canister. The sponge is covered with a sheet of moisture proof material which is metal or wax paper to prevent the tobacco within the canister from making direct contact with the moist sponge.
Guehler et al. U.S. Pat. No. 3,801,011 discloses a humidity control device for packages. The humidistat consists of a sheet-like material with a partially exposed layer of water containing micro-capsules adhered to it. The capsules have vapor transmitting walls which allow the water contained therein to vaporize and pass into the package.
McDonald U.S. Pat. No. 2,767,018 discloses a humidistat comprising a glass tube with an absorbent plug such as cotton at its open end. Water is contained in the tube and slowly volatilizes through the cotton plug to provide humidity to the surrounding enclosed environment.
McKee U.S. Pat. No. 1,871,419 discloses a humidifying sheet material for cigars. The sheet material carries humidifying agents in a dry condition which sheet material may be placed in direct contact with the cigars. The humidifying agent is moistened Glauber's salt, and the sheet material is tissue paper or fine mesh cloth.
The prior art also teaches various devices for dispensing volatile substances such as air fresheners and the like into an open (as compared to enclosed) environment.
Sullivan U.S. Pat. No. 4,529,125 discloses a device for dispensing volatile substances such as air fresheners and the like. Particularly, the device includes a pad having a reservoir layer for holding the volatile substance in its liquid phase and envelope layers on each side of the reservoir layer. The envelope layers are permeable to the volatile substance to allow for diffusion of the vapor through the envelope layers. The dispensing device is particularly designed to insure that the liquid phase of the volatile substance does not leak or come in contact with the environment and that the pad only dispenses the product in its vapor form. The volatile substance is incorporated directly into the reservoir layer while the dispensing device is being made, and the entire dispensing device is sealed into an air tight container so that the volatile substance does not volatilize and escape prior to being sold or used by the ultimate consumer.
Obermayer et al. U.S. Pat. No. 4,356,969 likewise discloses a vapor dispenser comprising a reservoir to contain a non-flowing liquid. The outer cover about the reservoir includes a membrane for vapor emission which emits the vapor at some predetermined rate. The reservoir itself is a gel-like material composed essentially of a liquid gel form which contains a volatile fragrance to be dispensed.
Sullivan et al. U.S. Pat. No. 4,158,440 discloses a device for releasing a volatile substance which includes a reservoir for storing the substance. The reservoir is surrounded by permeable material which has greater affinity for the volatile substance than the reservoir material has. An impermeable film is wrapped around the permeable material and sealed in order to retain the volatile substance in the reservoir until the dispensing device is opened for use. The reservoir may be filled at any time prior to the final sealing of the impermeable envelope. If the dispensing device is filled after manufacture, the reservoir material must be first impregnated with an expendable substantially nonvolatile substitute substance to prevent collapse of the reservoir material. The substitute substance is soluble in the volatile substance which will be added later in its place.
Sullivan U.S. Pat. No. 4,094,119 discloses a dispenser for a volatile substance including a reservoir and a permeable envelope. The volatile substance is supplied to the reservoir material prior to the reservoir material being encapsulated in the permeable envelope. Once the pad has been formed, the entire pad is sealed for distribution prior to use.
Engel U.S. Pat. No. 3,815,828 discloses a dispenser for dispensing volatile substances as a vapor. The dispenser includes a pad impregnated with an aqueous emulsion of the volatile substance. The pad is sealed in a liquid impervious but vapor permeable envelope. The pad is impregnated with the substance by immersing it in a bath of the aqueous emulsion.
O'Brien, J. Sci. Instruments, pp. 73-76 (March 1948) lists metal salts and equilibrium R.H. ranges. Additional listings will be known to those skilled in the art, such as CRC Handbook of Chemistry and Physics, 68th ed. (1987).
While the prior art discloses any number of ways for retaining a liquid substance in a reservoir for volatilization into the environment, none of the prior art patents addresses the problem presented by providing a humidistat in a package where the humidistat pad also functions as a spacer and where the humectant solution is added to the humidistat pad during the filling of the package on a high-speed production line.